1. Field of the Invention
This invention relates to thermoplastic compositions and to articles made by adhesively joining components. In particular, this invention relates to sprayable hot-melt adhesive compositions, methods of using said compositions to prepare articles, and to the articles so prepared.
2. Description of Related Art
Hot-melt adhesives are thermoplastic compositions which are solid at room temperature. When heated to liquid or molten form, the hot-melt adhesive can be applied to a substrate. If a second substrate is placed on the hot-melt adhesive before it cools back to a solid, an adhesive bond can be formed joining the two substrates. Hot-melt adhesives typically comprise a polymer, a tackifier, and a wax. See, for example, the descriptions of typical hot-melt adhesives and the function of each component in U.S. Pat. No. 5,026,756 (Arendt).
"Open time" is the time during which the adhesive is applied to a first substrate and remains sufficiently molten to effect a bond between the first substrate and a second substrate. Once the open time has been exceeded, the second substrate cannot be readily bonded to the first substrate. Long open times are generally preferred.
In hot-melt adhesive technology, spray application has been utilized in recent years. The advantages of spray application are described, for example, in European Patent Application 442,045 (Kehr et al.). Spray application generally permits contact-free deposition and ready application to uneven and irregular surfaces. Because spray application also allows for coating of less than the entire surface, spray application can reduce the amount of material used. Spray application is particularly useful where air or moisture permeability is desired. In addition, there may be less risk of thermal damage to substrates than with other application methods.
Kehr et al., supra, states that sprayable hot-melt compositions are generally based on thermoplastic rubber, such as styrenic block copolymers, or ethylene-vinyl acetate ("EVA") copolymers, or amorphous poly(alpha-olefin) copolymers. Kehr et al. goes on to state that each of these sprayable hot-melt compositions has some disadvantages. Compositions comprising thermoplastic rubber are said to be very expensive and to generally contain oil, which is undesirable in many cases. Compositions based on EVA copolymers are said to frequently exhibit an undesirably short open-time. Compositions based on amorphous poly(alpha-olefin) copolymers are said to generally not produce a uniform spray pattern because their rheological properties frequently cause the formation of large droplets on the substrate. These large droplets increase the probability of thermal damage to the substrate. Low molecular weight ("M.W.") poly(alpha-olefin) copolymers are less likely to form large droplets, however, these low M.W. polymers generally do not form acceptable bonds because of either low cohesive-strength or poor adhesive strength whereby an acceptable adhesive bond can usually not be achieved. In addition, Kehr et al. states that commercial amorphous poly(alpha-olefin) copolymers typically need to be sprayed at above 200.degree. C., which may result in damage to temperature-sensitive substrates.
Changes in diaper manufacturing have necessitated the development of new hot melt adhesives. See, for example, European Patent Application 451,920 (Chin). Diaper manufacturers now use thinner gauge polyethylene to manufacture diapers. Not only does thinner gauge polyethylene produce a more aesthetically pleasing diaper but it also represents significant cost savings. However, as the gauge of the polyethylene is decreased, the likelihood of burn through of these thin layers increases. In the past, the lower limit on the operating temperature was defined by the temperature at which the hot melt adhesive could be applied with sufficiently low viscosity for even and proper application. Such temperatures were generally in the range of 149.degree. C. to 177.degree. C. It would be beneficial if hot-melt adhesives could be applied at temperatures of from about 121.degree. C. to about 149.degree. C.
Styrenic block copolymers are widely used to make hot-melt adhesives for a variety of uses including diaper assembly. Hot-melt adhesives based on styrenic block copolymers such as KRATO.TM. thermoplastic rubber are said to provide excellent adhesion to non-polar substrates such as polyethylene which are commonly used in diaper manufacture. See, for example, Chin, supra. Additionally, these adhesives are said to maintain adhesion in elasticity demanding applications such as disposable diapers. Relatively low styrene content (from 15% to 24%) styrene-isoprene-styrene block copolymers are said to be useful in adhesives for such applications.
U.S. Pat. No. 4,497,941 (Aliani et al.) discloses wax-free hot-melt compositions comprising tackifier, and EVA copolymer having a melt index of 700 to 10,000. Said EVA copolymers are said to reduce the melt-viscosity in hot-melt systems, thereby replacing the wax of typical hot-melt compositions. See also, U.S. Pat. No. 4,613,632 (Aliani et al.). Said compositions are said to combine a long open time and a short setting time when applied in bead form.
Kehr et al., supra, describes sprayable hot-melt compositions comprising certain largely amorphous poly(alpha-olefin) copolymers. Said compositions are said to have rheology that is better suited for hot-melt spray application than prior art compositions. In contrast to prior art poly(alpha-olefin) copolymers, said compositions are said to have a "critical spray-temperature" below 180.degree. C. Specifically, certain terpolymers comprising interpolymerized units derived from ethylene, 1-butene, and propylene are disclosed.
Certain high-styrene block copolymers are described in S. Chin, "High Styrene Copolymers Meet Hot Melt Technology Needs," Adhesives Age, pp 26-30 (July 1991), in S. Chin, "New High Styrene Content Isoprene Based Styrenic Block Copolymers," TAPPI Notes - 1991 Hot Melt Symposium, pages 43-61, and in European patent publications 451,919 A2 and 451,920 A2 (S. Chin). Such copolymers are said to exhibit outstanding sprayability compared to current, commercially available, isoprene-based styrenic block copolymers.
U.S. Pat. Nos. 5,057,571 and 5,024,667 (Malcolm et al.) describe hot-melt adhesives comprising a radial block copolymer. Preferred block copolymers comprise polystyrene blocks and polybutylene or polyisoprene blocks.